1. Field of the Invention
The present invention relates to a vehicular headlamp structured to radiate light to form a low-beam distribution pattern. In particular, the present invention relates to a projector-type vehicular headlamp.
2. Description of the Related Art
Generally, a projection-type vehicular headlamp is structured such that a projection lens is disposed on an optical axis that extends in a vehicular longitudinal direction, a light source is disposed at a rear side of a rear side focal point of the projection lens, and light from the light source is reflected toward the optical axis by a reflector.
In addition, Unexamined Japanese Utility Model Application Publication No. Hei. 2-47704 (“Patent Document 1”) and Unexamined Japanese Patent Application Publication No. 2001-229715 (“Patent Document 2”) describe a side-insertion-type lamp configuration in which, in a projection-type vehicular headlamp with the structure described above, the light source is constituted by the light-emitting portion of a light source bulb that is fixedly mounted on the reflector from a side of the optical axis.
Moreover, the vehicular headlamp described in Patent Document 2 is structured such that a shade that blocks a portion of the light reflected from the reflector is provided in the proximity of the rear side focal point of the projection lens, thereby enabling radiation of light in order to form a low-beam distribution pattern.
If a side-insertion-type lamp configuration as described in either of the Patent Documents above is used, the longitudinal size of the lamp is reduced, thereby enabling the lamp to be made more compact.
However, the vehicular headlamps described in Patent Documents 1 and 2 have the following problems, which arise due to the fact that a light source bulb is fixedly mounted on a reflector with the light-emitting portion of the bulb lying in the same horizontal plane as that containing the optical axis of the lamp.
Specifically, although an area of the reflective surface of the reflector to the side of the optical axis is suitable for forming a diffusion region of a low-beam distribution pattern, if the light source bulb is fixedly mounted on the reflector in the same horizontal plane as the optical axis, the area to the side of the optical axis cannot be utilized effectively for light distribution control because a hole for inserting the light source bulb must be formed in the area to the side of the optical axis of the reflective surface, which makes it difficult to secure sufficient brightness for the diffusion region of the low-beam distribution pattern.
Furthermore, if the light source is structured as a line segment light source that extends in a bulb central axis direction, the luminous flux utilization factor with respect to light emitted from the light source is reduced in comparison with that which can be obtained from a line segment light source that extends in an optical axis direction, and the low-beam distribution pattern is darkened by a corresponding amount.